1. Field of the Invention
The present invention relates to an electronic apparatus for protecting a built-in semiconductor integrated circuit (IC) chip from an unexpected abnormal voltage applied from a DC power source such as an AC adapter.
2. Description of the Related Art
Conventionally, a power voltage to be applied to an IC chip is transformed and rectified from a commercial AC power source by an AC adaptor to form and supply a predetermined DC voltage. A voltage output from the AC adaptor is usually supplied stably. In the case in which an AC adaptor having a low performance is used or the case in which the AC adaptor is brought into a defective state, the generation of an unexpected overvoltage cannot be avoided. The overvoltage is supplied to the IC connected to the AC adaptor so that a circuit unit in the IC might be broken.
In order to avoid such a situation, conventionally as shown in FIG. 5, the voltage of an AC power source 1 is transformed and rectified by an AC adaptor 2 to form a predetermined adaptor output voltage Vad, then the adaptor output voltage Vad is stabilized by a regulator 3 having a high breakdown voltage, and said stabilized DC voltage is supplied to an IC 4.
On the other hand, an overvoltage limiting circuit comprising a resistor and an overvoltage limiting device (for example, a Zener diode) is connected to an input terminal in the IC into which a power is supplied from a three-terminal regulator so that the voltage limited by an overvoltage limiting device is obtained. There has been proposed an IC for supplying such a limited voltage to an internal circuit as disclosed OPI patent JP Hei 11-41801.
Concerning such a conventional type regulator in which the high breakdown voltage regulator needs to be provided between an AC adaptor and an IC, the production cost might be increased due to said high breakdown voltage regulator being separately required.
In case of using an overvoltage limiting circuit described in JP Hei 11-41801, a resistor constituting the overvoltage limiting circuit needs to be provided in the IC in addition to the overvoltage limiting device. Therefore, it is required for the IC process design to be carried out on the basis of an input overvoltage. Accordingly, due to such a restriction being placed on the design process, there might be a limitation of the overvoltage which the IC can cope with. Moreover, the resistor constituting the overvoltage limiting circuit is required to limit a current flowing into the overvoltage limiting device in a value within a predetermined range thereof. Furthermore, also required is to control generated heat, being generated in the resistor with depending on a current flowing in an internal circuit (or its unit block) to which a limited voltage is supplied, within a predetermined range. In order to satisfy these requirements, the degree of design freedom of the overvoltage limiting circuit is inevitably restricted.